Fix linker issues (@rpath + reexport)

Gphoto2 reexports libgphoto2 and libgphoto2_port dylibs. Ld is
unable to resolve @rpath-relative pathes but @loader_path-relative
ones are ok.

We fix pathes in Gphoto2 with install_name tool. Rpath-relative
in other libraries we leave as is for runtime convenience.